In the field of computer systems, significant advances have been made in recent years in providing affordable mass storage with increased storage capacity and decreased access time. Much of this effort has been directed at rotating magnetic medium, such as that found in hard disk drives. Unfortunately, access times with magnetic medium remain long, in the millisecond range.
In an effort to decrease access times and increase storage capacity, holographic storage systems have been developed. Examples of such holographic storage systems are described in U.S. Pat. No. 4,927,220, entitled "SYSTEM AND METHOD FOR PHOTO-REFRACTIVE HOLOGRAPHIC RECORDING AND SIGNAL PROCESSING", issued on May 22, 1990. That reference is herein incorporated by reference.
Holographic storage systems offer significant advantages over conventional mass storage systems. For example, the access time with holographic storage systems is on the order of microseconds. Furthermore, holographic storage systems retrieve arrays of data bits in parallel, rather than serially. For example, an array of 1,000 bits by 1,000 bits can be retrieved at once.
Normally, the dynamic range of the holographic storage medium is larger than that needed to store a single hologram with an acceptable signal-to-noise ratio. Therefore it is desirable to multiplex a number of holograms at one location to achieve higher storage capacity. One technique for multiplexing is referred to as planar (or .theta.) angular multiplexing, in which different holograms are stored at the same location by changing the relative planar angle of the reference beam, object beam, or the medium (the planar angle .theta. is the angle that measures rotation in the incident plane). Planar angular encoding is attractive because of its relative ease of implementation. In principle, beam deflecting systems or rotational stages can be used to accomplish planar angular multiplexing. To reduce complexity, however, usually the incident angle of only the reference beam is changed.
To prevent cross talk between holograms stored at the same location, each reference beam should be separated by the Bragg selectivity angle. The Bragg selectivity angle of the recording medium is a function of the thickness of the recording medium, among other things. In particular, the Bragg selectivity angle decreases as the thickness of the storage medium increases.
Therefore, the number of holograms that can be multiplexed using angle encoding depends on the Bragg selectivity angle, and on the ability of the system to change the angles of the reference or object beams. For example, for a system with an average Bragg selectivity angle of 1.degree. , and an optical system allowing 30.degree. of reference beam swing, only 30 different reference beam angles are available. This limits the number of holograms that can be multiplexed using planar angular multiplexing alone.
Therefore, a need has arisen for a method and apparatus that will allow for greater use of the dynamic range of the medium by greater multiplexing of holograms.